An image sensor comprises an array of pixel structures. The array can be operated with a rolling shutter or with a global shutter. For a rolling shutter, image acquisition is briefly shifted in time for every row of the pixel array. This leads to deformation of the scene. For a global shutter, all pixels are exposed together (synchronously). A global shutter is required to record fast moving subjects, or when the camera itself is moving. This is typically needed in high speed imaging applications or in machine vision. A global shutter pixel requires a memory element inside the pixel array, to store the captured image during the frame readout time. This stored pixel sample is then read out row-by-row while the next image is captured.
In some known global shutter pixel configurations, charges are stored in a potential well, in the charge domain. This charge storage node can unintentionally collect additional photoelectrons during a charge storage phase of operation. These additional photoelectrons are intended to be collected by the photodiode rather than by the storage node, but some charges will diffuse and be collected by the storage node instead. This results in a parasitic light sensitivity of the charge storage nodes.
FIG. 1 shows a global shutter pixel with in-pixel charge storage under a storage gate ø2. A cross-section is shown of the photodiode, charge transfer gates ø1, ø2 and ø3, the floating diffusion fd, the reset transistor RST, and the anti-blooming charge drain AB. The source follower and select transistor are shown on circuit level only. The photodiode is a pinned photodiode in which the surface is pinned at ground potential by a p+ surface implant layer. This p+ implant layer is connected to ground potential (not shown in the drawing). The diode itself is formed by an n-type implant under this pinning implant, which forms a junction with the p+ surface implant and the p-epitaxial layer. Charges are transferred from the photodiode to the storage gate ø2 via the transfer gate ø1 at the end of the exposure time. For readout, the floating diffusion fd is reset through RST, and then the charge is transferred from ø2 to fd by pulsing gates ø2 and ø3.
After the signal has been sampled under ø2, and while the signal is stored under ø2, the next image is acquired. Photo-generated electrons are created in the substrate. The electrons generated in the p-epitaxial layer are to be collected by the photodiode. Some are generated inside the depleted area of the photodiode and are immediately collected. Other charges are generated outside of this depletion area and will diffuse until they reach the electric field formed by the photodiode or by another junction or gate in the structure. Two such electrons e− are shown. One electron diffuses and is collected by the photodiode. Another electron however diffuses until it is collected by the storage gate ø2, which is biased at a high potential during storage. There is no electrical barrier for this electron to diffuse to this gate. A significant fraction of electrons are collected by this storage gate. A light shield 11 can be used to shield the storage node. This is only partially effective due to the random diffusion of the electrons.
FIG. 2 shows a known improvement intended to avoid charge diffusion to the storage nodes. This improvement is described, for example, in U.S. Pat. No. 6,225,670, provides a higher p dose under the storage node. The concentration difference between the p-well layer and p-epitaxial layer is sufficient to create a small potential barrier. The potential difference is given by:
  ϕ  =            kT      q        ⁢          ln      ⁡              (                              N            a            +                                N            a                          )            where Na and Na+ are the acceptor concentrations for the p and p+ regions, and kT/q is the thermal voltage (k=Boltzman constant, T=absolute temperature, q=elementary charge). Typical p-well concentrations are 1E17/cm3, while typical epitaxial layer concentrations are 5e14/cm3, forming a barrier of 134 mV at room temperature. FIG. 3 shows the electrostatic potential along the cross-sections A-A′, B-B′ and C-C′ in FIGS. 1 and 2. Another example of the use of a dopant profile to shield unrelated junctions in the pixel from charge collection is shown in US Patent application US 2007/0109437A1.
The present invention seeks a way to further improve shutter efficiency by reducing the parasitic light sensitivity of the pixel structure.